Draw another point between the destination and the nearest point on the shoreline. Suppose this point you just labeled is the point where the man should row to in order to optimize total time.
A = boat
B = nearest point on shoreline
C = destination
Z = point between B and C
The total travel time is divided in two parts: time traveled in water and time traveled on land.
Call AC = x, and try to express the sum of t_1 and t_2 in terms of x.
Here is the diagram for a similar problem with different conditions:
I hope this helps you get started on this question.